Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J11/00—Orthogonal multiplex systems, e.g. using WALSH codes
  • H04J11/0023—Interference mitigation or co-ordination
  • H04J11/0063—Interference mitigation or co-ordination of multipath interference, e.g. Rake receivers
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
  • H04L25/03006—Arrangements for removing intersymbol interference
  • H04L25/03159—Arrangements for removing intersymbol interference operating in the frequency domain
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/26—Systems using multi-frequency codes
  • H04L27/2601—Multicarrier modulation systems
  • H04L27/2647—Arrangements specific to the receiver only
  • H04L27/2649—Demodulators
  • H04L27/265—Fourier transform demodulators, e.g. fast Fourier transform [FFT] or discrete Fourier transform [DFT] demodulators
  • H04L27/2651—Modification of fast Fourier transform [FFT] or discrete Fourier transform [DFT] demodulators for performance improvement
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/26—Systems using multi-frequency codes
  • H04L27/2601—Multicarrier modulation systems
  • H04L27/2647—Arrangements specific to the receiver only
  • H04L27/2655—Synchronisation arrangements
  • H04L27/2662—Symbol synchronisation

Definitions

• the present invention relates generally to wireless communication systems, and in particular to a system and method for receiving and processing Orthogonal Frequency Division Multiplexing (OFDM) signals in a dispersive environment.
• OFDM Orthogonal Frequency Division Multiplexing
• OFDM is the radio access technology selected for a number of modern wireless communications systems, e.g., WiFi, 3GPP LTE, WiMax, and the like.
• the main idea of OFDM is to transmit a number of narrow-band symbols in parallel in the frequency domain, which are efficiently converted to and from the corresponding time-domain waveform using Inverse Fast Fourier Transform and Fast Fourier Transform (IFFT/FFT) operations.
• IFFT/FFT Inverse Fast Fourier Transform and Fast Fourier Transform
• a cyclic prefix may be introduced, by prepending a copy of the last part of the time-domain OFDM symbol to the beginning of that symbol prior to transmission.
• CP cyclic prefix
• a baseline OFDM receiver structure 10 is depicted in Figure 4.
• OFDM signals are received at one or more antennas 12, and processed by a front end receiver circuit 14, which may include low-noise amplification, frequency down-conversion, analog filtering, and the like.
• the signal is digitized by an analog-to-digital converter (ADC) 16, and baseband filtered by a digital filter 18.
• a timing reference is established, for example using the CP properties or known synchronization signals, and provided to a Fast Fourier Transform (FFT) processor 20.
• FFT Fast Fourier Transform
• a length-N sample sequence, starting at the timing reference, is then processed by the FFT 20.
• the individual carriers are de-rotated to undo the per-carrier channel impact in channel estimation block 22, and the transmitted symbols are recovered by symbol detector 24.
• the resulting ISI may not be negligible, despite the degree of robustness built into the OFDM scheme. This result may occur even when the CP is applied.
• the length of the CP in a practical system is limited, since it is chosen as a compromise between providing protection in "typical” scenarios and minimizing the "wasted" transmission energy that does not directly improve the available data rates or coverage.
• SFN Single Frequency Network
• CoMP Coordinated Multi-Point
• a received OFDM signal is processed to determine a plurality of reference delays, which may include the path delays of a multipath channel, as well as other delays not corresponding to the propagation paths.
• the effective channel estimates corresponding to each reference delay are determined, as is the covariance of the ISI and noise components observed at each delay.
• Combining weights resulting in maximum post-combining SINR are determined for all subcarriers.
• a corresponding plurality of FFTs is applied to the incoming sample stream, one at each of the reference delays.
• the individual subcarriers from each FFT output are then combined using the combining weights. This produces a single FFT output with suppressed ISI, which may be used in all further processing.
• the multiple FFT outputs may be combined in later stages of baseband processing.
• One embodiment of the invention relates to a method of processing a received OFDM signal in a wireless communication receiver.
• the OFDM signal is received at one or more antennas. At least two reference delays are determined per OFDM symbol in the received signal.
• the received signal is FFT processed at each determined reference delay, to generate at least two sets of frequency domain samples. Channel and interference estimates are generated for each set of frequency domain samples. Combining weights are determined, based on the channel and interference estimates, to generate the maximum post-combining SINR.
• the at least two sets of frequency domain samples are combined using the combining weights, and symbols are detected from the combined frequency domain samples.
• Another embodiment of the invention relates to an OFDM receiver.
• the receiver includes one or more antennas operative to receive an OFDM signal and a timing control unit operative to generate a plurality of FFT processing reference delays.
• the receiver also includes an FFT processor operative to FFT process the received signal at two or more reference delays, to generate two or more sets of frequency domain samples, in response to the timing control unit.
• the receiver further includes a channel and covariance estimator operative to generate channel and covariance estimates for each set of frequency domain samples, and a symbol detector operative to generate combining weights that yield a maximum post-combining SINR, and combine the sets of frequency domain samples based on the combining weights to detect OFDM symbols.
• Figure 1 is a timing diagram depicting a conceptual approach to OFDM processing according to embodiments of the present invention.
• Figure 2 is a flow diagram depicting a method of processing OFDM signals in a dispersive environment according to embodiments of the present invention.
• Figure 3 is a functional block diagram of an OFDM receiver for processing OFDM signals in a dispersive environment according to embodiments of the present invention.
• Figure 4 is a functional block diagram of a prior art OFDM receiver.
• the received signal will be a sum of the multiple scaled and shifted copies plus noise:
• embodiments of the present invention perform FFTs at different reference delays and account for the fact that the interference components will be correlated. This offers redundancy which may be used to suppress the undesired signal components by the interference rejection combining (IRC) principle.
• IRC interference rejection combining
• the per-path ISI term has the following structure: >o
• G k and R k are outlined below.
• a practical receiver may compute the weights by solving the required LSE using a variety of known techniques, or by explicitly inverting the covariance matrix:
• the receiver may correlate to known synchronization signals, e.g., P-SCH, S-SCH in the case of a 3GPP LTE system, to determine the path delays ⁇ m .
• the medium channel coefficients g m may then be found by computing the IDFT of samples of interest of the frequency domain channel estimates.
• the peak positions may be found by performing a full IFFT of the channel estimates in the frequency domain and detecting the peaks.
• the covariance matrix may be decomposed according to the form
• the covariance may be estimated via the construction above, with the scaling parameters E c and N 0 known from other receiver processing stages, or estimated using known parameter estimation routines. Alternatively, the covariance may be estimated blindly (non-parametrically) from the data, using the pilot symbols from the FFTs corresponding to each reference delay.
• FIG. 2 depicts a method 100 of receiving and processing OFDM signals
• Figure 3 depicts a receiver architecture 130 for practicing the method 100.
• OFDM signals are received at one or more antennas 132 (step 1 10).
• the received signals are processed in a front end receiver circuit 134, which may include, e.g., low-noise amplification, frequency down-conversion, analog filtering, and the like.
• the signal is digitized by an analog-to-digital converter (ADC) 136, and baseband filtered by a digital filter 138.
• a plurality of reference delays for each OFDM symbol from each antenna 132 which may for example take the form of a Path Delay Profile, is determined in a time synchronization circuit 142 (step 1 12).
• the PDP may determined, e.g., by correlation to synchronization signals or reference signals.
• the PDP is received by a control unit 144 that determines FFT sampling instances.
• the received signal is processed by an FFT processor 140 at the determined FFT timing instances (step 1 14).
• the resulting sets of frequency domain samples for all sub- carriers are stored in a buffer, and utilized by a channel and covariance estimation unit 146 to produce channel estimates for each set of frequency domain samples (step 1 16).
• FFT processing and channel estimation is performed for all identified FFT sampling delays (steps 1 18, 120).
• the block 146 additionally calculates covariance estimates for each set of frequency domain samples (step 122), which are correlated with the other sets of frequency domain samples.
• Combining weights are formed (step 124). In one embodiment of the invention, the combining weights are based on the channel and interference estimates, and may be, for example, the weights that will yield the maximum post-combining SINR. All of the sets of frequency domain samples are combined using the combining weights (step 126). The symbols are then detected from the combined signal in the symbol detector block 148 (step 128).
• the placement of the reference delays may be a superset of detected path delays, with additional reference delays chosen to provide useful information for ISI suppression.
• Embodiments of the present invention may be applied as a pre-processing step to a state-of-the-art OFDM receiver, where all subsequent baseband processing remains unchanged.
• the extended set of FFT outputs may be made available to the equalization and/or spatial combination stages, replacing the original N rx -element operations with their larger equivalents, using the described covariance relationships.
• Embodiments of the present invention exhibit some conceptual similarities to the GRAKE receiver structure applied to WCDMA. Accordingly, a number of extensions and variants developed in the GRAKE context are applicable to embodiments of the present invention, and may provided improved performance and/or computational simplicity. These include, e.g., techniques of reference delay selection; fitting parameter estimation; numerically robust combining weight computation; and the like.
• Embodiments of the present invention provide OFDM operation with improved robustness in heavy multipath without requiring excessively long CPs. Indeed, embodiments of the present invention may allow operation of OFDM without the use of CPs, eliminating the overhead of transmitting the CP over the air interface. Suppressing ISI removes (or at least elevates) the achievable effective SINR ceiling at higher geometries. Improved user throughput, high-rate coverage, and/or network capacity is achieved as a result.
• Figure 5 depicts some examples of ISI suppression via interference rejection combining (IRC) on two-tap multipath channels, for both single- and dual-antenna receivers.
• the low-dispersion channels have paths at 2 and 4, and 4 FFTs at sampling instances 0, 2, 4, and 6 are combined.
• the high-dispersion channels have paths at 20 and 40, and the FFT sampling instances are 0, 20, 40, and 60. Improvement is seen in all medium-to-high geometry scenarios.
• any or all of the functional blocks may be merged, and/or functionality included in one block may be separated into two or more functional blocks.
• the method steps depicted in Figure 2 may be merged or separated, and one or more steps may be omitted and/or additional steps added, in any particular implementation.

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Physics & Mathematics (AREA)
• Discrete Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Mathematical Physics (AREA)
• Power Engineering (AREA)
• Noise Elimination (AREA)
• Radio Transmission System (AREA)

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• 2009
  • 2009-02-17 US US12/371,971 patent/US8275074B2/en not_active Expired - Fee Related
• 2010
  • 2010-02-16 WO PCT/EP2010/051896 patent/WO2010094672A1/en active Application Filing
  • 2010-02-16 EP EP10705138.5A patent/EP2399372B1/de not_active Not-in-force
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